1. Field of the Invention
The present invention relates to a high speed tool steel excellent in cold strength, wear resistance and in hardenability and also to a method for manufacturing such high speed tool steel. More particularly, the present invention relates to a high speed tool steel particularly excellent in hot strength and in toughness with a minimum variation in tool performance when used as a material for: a metallic mold used for forming plastics; and, a swaging tool, for example such as a press forming die, a press forming punch and like tools.
2. Description of the Related Art
Heretofore, widely used as materials in production of: a tool such as a press forming punch used in hot precision press working; and, a metallic mold used for forming plastics, are those excellent in hot strength or toughness, for example such as: a hot working tool steel of the type “AISI H19”; and, a high speed tool steel of the type “AISI M2”. However, these conventional types of tool steels are still poor in toughness and like mechanical properties. This often leads to breakage and occurrence of heat cracks of a tool product made of the conventional types of tool steels in use.
More particularly, in case of the former steel (i.e., hot working tool steel), this type of steel is low in carbon content and therefore low in cold strength. Due to this, the former steel often suffers from its poor resistance to fatigue and poor wear resistance together with its breakage in use.
On the other hand, in case of the latter steel (i.e., conventional type of the high speed tool steel), the applicant of the subject Patent application has previously proposed, in Japanese Patent Laid-Open application No. H02-8347 (Laid open in 1990): a high speed tool steel, which is improved in cold/hot strength and toughness so as to improve a product made of this type of steel in crack resistance and in resistance to fatigue at high temperatures in use. The product made of this type conventional tool steel is excellent in tool performance. On the other hand, in order to realize the mass production of such product made of the tool steel, it is necessary to produce a large-sized steel ingot. However, such large-sized ingot often varies in composition of its carbides. Due to the presence of variations in composition of the carbides, the product made of the tool steel obtained from the large-sized steel ingot often varies in tool performance even when the product is sufficiently controlled in quality during its production processes.
Also proposed by the applicant in another Japanese Patent Laid-Open application No. H04-111962 (Laid open in 1992) is a method for manufacturing a high speed tool steel. This method employs a conventional electro-slag melting process to reduce anisotropy in mechanical properties of a tool product made of the tool steel, and improves the product in tool life. However, the product made of the tool steel is still poor in toughness in use.